Molten metal, particularly molten aluminum, in practice generally contains entrained solids which are deleterious to the final cast metal product. These entrained solids appear as inclusions in the final cast product after the molten metal is solidified and cause the final product to be less ductile or to have poor bright finishing and anodizing characteristics. The inclusions may originate from several sources. For example, the inclusions may originate from surface oxide films which become broken up and are entrained in the molten metal. In addition, the inclusions may originate as insoluble impurities, such as carbides, borides and others or eroded furnace and trough refractories.
It is naturally highly desirable to improve the filtration of molten metals in order to remove or minimize entrained solids in the final cast product, particularly with respect to molten aluminum and especially, for example, when the resultant metal is to be used in a decorative product, such as decorative trim or sheet made from the 5000 series of aluminum alloys, as aluminum Alloys 5252 and 5657. Inclusions as aforesaid cause loss of properties in the final solidified alloy and lead to degradation of processing efficiency and loss of properties in the final product. For example, one type of finishing flaw which is particularly significant in decorative trim or sheet made from aluminum Alloy 5252 is a stringer defect known as a linear defect.
Rigorous melt treatment processes, such as gas fluxing, minimize the occurrence of such defects; however, these are not successful in reducing them to a satisfactory level for critical applications. Conventionally, melt filtration is utilized in order to decrease the extent of such defects, and others caused by the presence of inclusions in the melt. The most common form of melt filtration involves the use of open weave glass cloth screens placed in transfer and pouring troughs or in the molten pool of metal in the top of a solidifying ingot. Such filters have been found to be only partially effective since they remove only the larger inclusions. Another type of filter in common use is a bed filter made up, for example, of tabular alumina. Such filters have many disadvantages, perhaps the most serious of which is the great difficulty experienced in controlling and maintaining the pore size necessary for efficient filtration. Another difficulty with such filters is their tendency to produce an initial quantity of metal having poor quality at the start up of each successive casting run. This behavior results in a so-called ingot "butt effect", that is, ingots having butt portions of relatively poor quality which must be scrapped and recycled. Still further, the metal in a bed filter must be maintained molten even when the filter is not in use.
Porous ceramic foam materials are known in the art, for example, having been described in U.S. Pat. Nos. 3,090,094 and 3,097,930. These porous ceramic foam materials are known to be particularly useful in filtering molten metal, as described in U.S. Pat. No. 3,893,917 for "Molten Metal Filter" by Michael J. Pryor and Thomas J. Gray, patented July 8, 1975, and also as described in copending U.S. patent application Ser. No. 563,213 for "Ceramic Foam Filter" by John C. Yarwood, James E. Dore and Robert K. Preuss, filed Mar. 28, 1975.
Porous ceramic foam materials are particularly useful for filtering molten metal for a variety of reasons included among which are their excellent filtration efficiency, low cost, ease of use and ability to use same on a disposable, throwaway basis. The fact that these ceramic foam filters are convenient and inexpensive to prepare and may be used on a throwaway basis requires the development of means for easily and conveniently assembling and removing porous, molten metal filters from a filtration unit while providing a highly efficient filtration assembly.
Accordingly, it is a principal object of the present invention to provide an improved method and apparatus for the filtration of molten metal with a removable filter plate.
It is a particular object of the present invention to provide an improved removable filter plate for use in the filtration of molten metal.
It is a still further object of the present invention to provide improvements as aforesaid which are convenient and inexpensive to utilize and which result in high filtration efficiency.
Further objects and advantages of the present invention will appear hereinbelow.